Radio frequency (RF) transmitters of the type used in mobile wireless telephones (also known as cellular telephones) and other portable radio transceivers commonly include transmit power control circuitry that adjusts the power of the transmitted RF signal. The power control circuitry can adjust a linear power amplifier to increase or decrease the transmitted RF power in response to operating conditions. For example, in some instances it may be desirable to maintain a constant transmitted RF power despite changes in operating conditions, such as power supply voltage, temperature and RF load. In other instances it may be desirable to maintain transmitted RF power as low as possible and boost it only when necessary to maintain adequate performance.
It is known that power supply fluctuations can hamper power control. The power supply circuitry in a portable radio transceiver is typically battery-based. Like any battery-powered device, operation of the mobile telephone or other such portable radio transceiver gradually discharges or drains the battery. As battery power decreases, the transmitter's ability to properly transmit a signal at the requisite RF power level diminishes. More specifically, if the power control circuitry requires high transmission power at a time when battery power is low, the power amplifier can undesirably distort the transmitted signal. Such distortion is typically measured as error vector magnitude (EVM).
To minimize the above-referenced distortion and thus improve EVM, circuitry has been employed that causes the transmit power control circuitry to adjust the power amplifier to decrease transmitted RF power in response to a decrease in battery power. Although the resulting transmitted RF power may be lower than the power control circuitry would otherwise dictate for the operating conditions, transmitted signal distortion can be reduced. It is generally believed preferable to transmit a lower-power RF signal having minimal distortion than a higher-power RF signal having more distortion.